We are proposing a broadly based approach to the problem of chromatin structure in Dictyostelium discoideum as it relates to biological function. Dictyostelium is an organism ideally suited for these studies since in growing cells 19 percent of the single copy genome is expressed as mRNA and 53 percent as nuclear Hn RNA. At one stage of differentiation there is the expression into mRNA and nuclear RNA of an additional 11 percent and 26 percent of the genome respectively. There are three main parts to the program of proposed research. Part I will include studies on the basic structure of chromatin from growing cells. The histone population will be characterized with particular attention to variant forms differing in primary base sequence or post translational modifications. The more prominant non-histone chromosomal protein associated with the various forms of mono and oligo nucleosomes will also be characterized. Endonucleases will be used to study both the nucleosomal repeat length for bulk chromatin and for individual genes of developmental interest as well as to identify sites of interaction between DNA and histones. In the second part we will use the simple, but well characterized pattern of developmental gene expression in Dictyostelium to correlate heterogeneity in chromatin structure with the biological function of the DNA. Both recombinant DNA and cDNA probes for genes expressed under developmental control or for DNA which is either transcribed throughout development or transcriptionally inert will be used to identify variant nucleosomal species which may show specific association with transcriptionally active, developmentally regulated, or inert DNA. In addition we will also look for developmentally specific changes in the histones, non-histone chromosomal proteins or other features of nucleosome structure. Use will also be made of DNAse I digestion as a probe of active gene structure to ask whether the co-ordinate expression of 25 to 30 percent of the genome late in development involves a structural transition of the corresponding chromatin segments. Finally, the higher order structure of chromatin associated with individual genes will be examined.